This invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device of the active matrix type in which the axis of liquid crystal molecules is controlled by applying an electric field substantially in parallel with a substrate inside of a surface of the display panel, thereby providing a wide viewing angle and a high image quality simultaneously. This invention also relates to a liquid crystal display device having spacers of a novel constitution which assure a constant distance between a pair of substrates between which a liquid crystal composition is provided.
Liquid crystal display devices have been widely used as display devices for notebook type computers and computer monitors which are capable of producing a color display of high definition.
Such liquid crystal display devices are roughly classified into two systems. In the liquid crystal display device of one system (a simple matrix type liquid crystal device), a liquid crystal composition is substantially sandwiched between opposing faces of at least two substrates with at least one substrate being made of transparent glass, thus constituting a so-called liquid crystal panel, and a voltage is selectively applied to various pixel forming electrodes formed on the substrate of the liquid crystal panel so as to turn on or off given pixels. In the liquid crystal display device of the other system (active matrix liquid crystal display device), the above-mentioned various electrodes are provided with active elements for selecting pixels and given pixels are turned on or off by selecting these active elements.
The active matrix liquid crystal display device is represented by a liquid crystal display device which uses thin film transistors (TFT) as active elements. The liquid crystal display devices using the thin film transistors have been widely used as display terminal monitors of OA equipment, since the devices are thin and light-weight and exhibit a high image quality equivalent to that of cathode ray tubes.
In view of the difference in the method of driving the liquid crystal, the display system of the liquid crystal display devices is roughly classified into two types. In one display system, the liquid crystal composition is sandwiched by two substrates constituted by transparent electrodes and is driven by a voltage applied to the transparent electrodes, and light incident upon the liquid crystal composition layer after being transmitted through the transparent electrodes is modulated to provide the display. Most of the products that are now available have adopted this system.
Another one is a system in which the liquid crystal composition is driven by an electric field which is nearly parallel with the surface of a substrate, being generated between two electrodes that are formed on the same substrate, and light incident upon the liquid crystal composition layer through a gap between the two electrodes is modulated to provide a display. This system is characterized by its remarkably wide viewing angle and is an extremely prominent system in connection with the active matrix liquid crystal display device. Features of the latter system have been disclosed in, for example, Japanese Publication of the Translation of International Patent Application No. 505247/1993, Japanese Patent Publication No. 21907/1988 and Japanese Patent Laid-open No. 160878/1994. Hereinafter, the liquid crystal display device of this system will be referred to as a transverse electric field liquid crystal display device.
FIG. 17 is a cross sectional view of an essential part of a display panel showing how an electric field is generated by the transverse electric field liquid crystal display device. In this liquid crystal display device, a video signal line DL, a counter electrode CT and a pixel electrode Px are formed on one substrate SUB1. The liquid crystal display device includes an orientation control layer ORI1, which is formed on the interface between a protective film PSV, formed on upper layers of the video signal line DL, the counter electrode CT and the pixel electrode PX, and a layer of a liquid crystal composition LC. On the other substrate SUB2, there are color filters FIL, defined by a black matrix BM, an overcoat film OC, which is formed such that it covers upper layers of these color filters FIL and the black matrix BM and is formed so as to prevent the constituting members of color filters and the black matrix from affecting the liquid crystal composition (hereinafter called liquid crystal simply), and an orientation control layer ORI2 which is formed on an interface between the overcoat layer OC and the layer of the liquid crystal LC.
Insulation films GI and AOF are formed on the one substrate SUB1, a video signal line DL is made of conductive films d1 and d2, a counter electrode CT is made of a conductive film g1, and the pixel electrode PX is made of a conductive film g2.
The distance between the pair of substrates SUB1, SUB2 (the thickness of the layer of the liquid crystal: cell gap) is, in general, set to a given value by arranging spherical spacers (not shown in drawings) in a distributed manner between both substrates. Although polarizing plates are provided on the outer surfaces of the substrate SUB1 and the substrate SUB2, they are omitted from the drawings.
Although no relevancy is found with the transverse electric field liquid crystal display device, Japanese Patent Laid-open No. 73088/1997 discloses the use of conical spacers which are fixedly formed on a protective film of a color filter substrate in place of such spherical spacers, or conical spacers, which are fixedly formed by laminating color filters.
In FIG. 17, the orientation direction of molecules of the liquid crystal LC is controlled in response to the electric field, which is generated between the pixel electrode PX and the counter electrode CT nearly in parallel with the substrates so as to provide, an image display. Here, however, an electric field which does not contribute to the display is generated between the video signal, line DL and the counter electrode CT. In case the gap between these electrodes is too narrow, the intensity of the electric field between the video signal line DL and the counter electrode CT is strengthened, and, hence, the liquid crystal LC is driven and an undesirable light is transmitted through the liquid crystal.
Although the black matrix BM is positioned in a region defined between these electrodes, in case the optical density of the black matrix BM is low, the above mentioned transmitting light cannot be shielded completely, and, hence, a leakage of light occurs. This leakage of light gives an adverse influence on the quality of the display, such as the lowering of the contrast or the occurrence of crosstalk. To solve such problems, it becomes necessary to increase the distance between the above-mentioned electrodes or to increase the optical density of the black matrix BM. Although FIG. 17 shows the case where the video signal line DL and the counter electrode CT are arranged close to each other, similar problems also occur in case the video signal line DL and the picture electrode PX are arranged close to each other.
FIG. 18 is a schematic cross sectional view showing an example of an arrangement of electrodes which form one pixel of a liquid crystal panel constituting a conventional transverse electric field liquid crystal display device. On an active matrix substrate SUB1, there is a pixel electrode PX, counter electrodes CT1, CT2, a drain electrode SD2, an insulation film PSV, an orientation film ORI1 and the like, while on a color filter substrate SUB2, there are a black matrix BM, color filters FIL(R), FIL(G), FIL(B) (not shown in drawings), an overcoat film OC, an orientation film ORI2 and the like. A liquid crystal LC is filled in a gap defined between opposing faces of both substrates. In the drawing, POL1, POL2 denote polarizing plates, E denotes an electric field for selecting a pixel, and Exe2x80x2 denotes an undesired electric field.
Upon selection of this pixel, the electric field E is generated between the pixel electrode PX and the counter electrode CT1 and the orientation of the liquid crystal LC is controlled in response to this electric field E and, hence, a so-called lighting condition is obtained. Here, however, there also exists a potential difference between the pixel electrode PX and the neighboring counter electrode CT2 so that an undesired interference electric field Exe2x80x2 is generated between them. As a result, a proper electric field is not applied to the liquid crystal of the selected pixel.
A method for applying a proper electric field to the liquid crystal, while preventing electric fields applied in parallel with the substrate from interfering with each other, and corresponding means for achieving a high quality image are disclosed in, for example, Japanese Laid-open No. 160878/1994 and Japanese Laid-open No. 120061/1997.
As described above, in the transverse electric field liquid crystal display device, a large number of wirings are formed on the active matrix substrates and various electric signals are applied to them with the result that the electric fields interfere with each other. Accordingly, the electric fields applied to the liquid crystals are affected by the undesired electric fields and there arises a case wherein the proper electric fields are not applied to the liquid crystals.
Furthermore, due to an undesired electric capacity which is generated between the electrodes, the voltage applied to the liquid crystal tends to fluctuate. Such a phenomenon becomes a cause of the deterioration of the display quality of the liquid crystal display device.
In particular, the electric field generated by a video signal electrode, which transmits a signal to each pixel having an active element, such as a thin transistor film TFT, affects the electric field between the pixel electrode which operates the liquid crystal and the common electrode. The potential of the video signal electrode is propagated by the image signal and, hence, the potential is always fluctuating during the frame period. For example, in case the pixel electrode (held in a floating condition when the active element is turned off) is arranged close to the video signal electrode, irregularities such as striped shape in parallel with the video signal electrode called smear, which is a crosstalk phenomenon, are generated by the fluctuating potential of the video signal electrode.
To suppress this phenomenon, there has been a technique which arranges a common electrode to which the potential is always applied from the outside as the closest electrode to a video signal electrode (see Japanese Patent Laid-open No. 46916/1994). However, the provision of only this technique is less than optimal to obtain a sufficient electric field shielding effect, and so the problem of smear still exists.
It is estimated that, in a case where the wiring width of the common electrode is widened, the shielding effect is increased and, hence, the smear may be suppressed. The widening of the wiring width, however, brings about a lowering of the aperture rate of the longitudinal display device. To cope with this, Japanese Patent Laid-open No. 120061/1997 discloses a technique in which a light shielding film is provided in parallel with a video signal electrode and this light shielding film is made of a conductive material.
However, portions which are usually formed as light shielding films on the color filter are not constituted merely by portions which are in parallel with the video signal electrode. In the transverse electric field system, there exists a problem in that, in case the conductivity of the light shielding film disposed right above the electric field is high, the electric field applied between the video signal electrode and the common electrode is absorbed by the light shielding film. In an actual fabrication, it is necessary to provide two kinds of light shielding films, wherein one film constitutes a portion which is to be conductive and the other film constitutes a portion which does not need to be conductive. Accordingly, the fabrication steps of the color filter become complicated.
In the liquid crystal display device, it is important to maintain the cell gap (d) between the opposing substrates of the liquid crystal panel at a constant value. In the conventional liquid crystal display device, this is ensured by scattering a large number of spacers, for example, transparent spherical bodies, between the substrates.
When the cell gap xe2x80x98dxe2x80x99 is changed at respective positions of the surface of the substrate, the retardation xcex94nxc2x7d (product of the refractive index anisotropy xcex94n and the thickness of the liquid crystal layer, that is, the cell gap d) is changed. This change of the retardation xcex94nxc2x7d gives rise to a change in the optical speed of response and the transmission factor of the liquid crystal. Accordingly, when the cell gap xe2x80x98dxe2x80x99 at respective positions of the surface of the substrate is not uniform, the contrast ratio and the chromaticity of the display screen are changed, thus resulting in a lowering of the display quality, which cannot assure the required uniformity of the screen.
To obtain a uniform cell gap, the amount of scattered spacers may be increased. In this case, however, the dispersion of the spacer is difficult and hence, some spacers become coagulated, thus forming a lump, and such lump is scattered, which brings about a display of poor quality. Accordingly, the amount of spacers which can be increased is limited.
Furthermore, in the scattering method, the spacers are arranged at random positions and, hence, they are located at various concaved and convexed portions on the substrate so that there exists a limit in obtaining a uniform cell gap. Still furthermore, the arrangement of the liquid crystal molecules in the vicinity of the spacers is disturbed and, hence, there is a problem that a leakage of light occurs at portions where the arrangement of the liquid crystals is disturbed due to the spacers present in the display region and the black level is lowered due to this leakage of light, thus lowering the contrast.
Furthermore, in case spacers are used, it is required that the thickness of the liquid crystal layer is made uniform in the display surface. In case the thickness is not made uniform sufficiently, it gives rise to irregularities in the luminance in the inside of the display screen due to the irregularities of the thickness of the liquid crystal layer. In forming columnar spacers, it is difficult to form the columnar spacers while making their height uniform. This is due to the method of forming the columnar spacers. In general, the columnar spacers are formed by coating a photosensitive resist on the color filter or the TFT substrate, exposing the resist with the use of a mask and developing the resist, and, hence, the columnar spacers are affected by the coating irregularities, the lighting irregularities and the developing irregularities.
Furthermore, in case the distance between electrodes is widened so as to reduce the intensity of the electric field between the video signal line DL and the counter electrode CT or between the video signal line DL and the pixel electrode PX, the display pixel region or domain inevitably becomes narrow and, hence, the aperture rate is lowered, resulting in a lowering of the luminance and an increase of the power consumption.
On the other hand, there exists a further problem in enhancing the optical density of the black matrix BM. In the transverse electric field liquid crystal display device, the black matrix BM must have a high resistance (see Japanese Patent Laid open No. 43589/1997, for example). This is because the electric characteristics of the black matrix BM affects the formation of the transverse electric field nearly in parallel with the substrate, such that, in case the resistance of the black matrix BM is low, an ideal transverse electric field cannot be formed and problems such as the lowering of the luminance, the lowering of the contrast and the narrowing of the viewing angle occur. To make the black matrix BM have a high resistance, it is desirable to use a pigment dispersed resin resist. Here, however, in case the pigment concentration ratio in the resist is increased so as to increase the optical density of the black matrix BM, the density of the resin is decreased and, hence, the processing ability of the photolithography is deteriorated.
To be more specific, it gives rise to problems such as the lowering of resolution, the lowering of developing margin, and the prompt generation of pigment residue. Furthermore, in case the film thickness of the black matrix BM is made thick so as to increase the optical density, the flatness of the color filter is deteriorated, the rubbing characteristics of the orientation control layer ORI2 is deteriorated, and it becomes difficult to make the thickness of the liquid crystals LC (so-called cell gap) uniform resulting in a poor display quality, such as the deterioration of the speed of response.
Accordingly, it is an object of the present invention to provide a transverse electric field liquid crystal display device in which the above-mentioned problems of the prior art are solved and which is capable of providing an image display of high quality by eliminating the leakage of light caused by spacers and suppressing the disturbance of the electric field between the electrodes.
It is another object of the present invention to provide a liquid crystal display device which can minimize the irregularities of luminance in the inside of a display screen, can prevent lowering of the aperture rate, and can prevent a lowering of the luminance of contrast and the occurrence of crosstalk even when a black matrix of relatively low optical density is used.
Still furthermore, the present invention is characterized by the following constitutions.
In the liquid crystal display device comprising a liquid crystal panel in which at least two kinds of color filters of different colors and a black matrix interposed between respective color filters are provided on one of a pair of substrates with at least one substrate being transparent, and a group of electrodes for selecting pixels is provided on the other of a pair of the transparent substrates, and which further includes a liquid crystal layer made of a liquid crystal composition sandwiched between the pair of substrates and having the dielectric anisotropy, a polarizing plate laminated on an outer surface of at least one of a pair of the substrates, and drive means which applies drive voltages for display to a group of the electrodes, the improvement is characterized in that the liquid crystal display device includes spacers which bridge the gap defined between the opposing inner surfaces of a pair of the substrates, and the specific resistance of one substrate side of the spacers is smaller than the specific resistance of the other substrate side of the spacer.
The spacers are formed by joining top faces of protrusions respectively formed on a pair of the substrates. The specific resistance of one substrate side of the spacer is set to less than 108 xcexa9xc2x7cm and the specific resistance of the other substrate side of the spacer is set to 108 xcexa9xc2x7cm. The protrusion of one substrate side of the spacer is formed of metal or organic polymer material containing carbon particles and the protrusion of the other substrate side of the spacer is formed of organic polymer based, insulation material.
The spacers according to the present invention are continuously-formed partition-like members or one or a plurality of columnar members which are formed on the side face of the pixel region along the matrix of the black matrix and right below the black matrix. The attrition-like or columnar spacers are constituted by fixedly joining the top faces of the protrusions which are formed on the active matrix substrate and the color filter substrate. The spacers are formed using the process for forming insulation films or the like on the inner surfaces of these substrates. The spacer is provided with a portion at the protrusion side of the color filter substrate side which has a smaller specific resistance than the protrusion of the active matrix substrate side.
Due to the above-mentioned respective constitutions, the spacer can be used also as a member for absorbing an undesired interference electric field and, hence, the provision of only a single layer of light shielding film is sufficient. Furthermore, the height of the spacer at the color filter substrate (one substrate) side is arbitrarily adjustable and, hence, the control of an absorption amount of electric field is facilitated. Still furthermore, since the scattering of spherical spacers which has been necessary conventionally is no longer necessary, no problem of leakage of light occurs.
In contrast to the scattering of the conventional spherical spacers, the spacers of the present invention can be formed at given positions except for the pixel region and, hence, the control of the cell gap is facilitated. Furthermore, by making top faces of the protrusions formed on respective substrates have a planar structure, the contact areas of opposing substrates can be increased and, hence, the pressure applied to the spacers can be dispersed, thus enhancing the uniformity of the cell gap. Still furthermore, the spacer has a two-layered structure and, hence, the height of the respective protrusions can be restricted to less than the height of the cell gap, thus facilitating the rubbing of the orientation film and reducing the irregularities of the orientation around the periphery of the spacer.
Although the member for absorbing an undesired electric field may be a conductive body, such as metal, in such a case, the conductive body is arranged only at the facing portion of the pixel electrode (video signal electrode) and a light shielding film having a large insulation ability is coated on the conductive body such that the film covers the conductive body. Here, the conductive body is constituted to have the same potential as the counter electrode and, hence, the electric field shielding effect is further enhanced.
Furthermore, as the insulating light shielding film, an organic polymer material into which conductive particles made of metal, carbon or the like are mixed is used and the specific resistance can be regulated by adjusting the amount of the conductive particles used in the mixture.
The liquid crystal display device comprises a liquid crystal panel which includes a pair of substrates, with at least one of them being transparent, at least two kinds of color filters of different colors for color display and a black matrix interposed between the respective color filters, which are formed on one of a pair of the transparent substrates, a group of electrodes formed on a pair of the transparent substrates, a layer made of a liquid crystal composition disposed between a pair of the substrates and having a desired dielectric anisotropy, an orientation control layer for causing the molecular arrangement of the layer of this liquid crystal composition to be arranged in a given direction, and drive means which applies drive voltages to a group of the electrodes, and columnar spacers containing particles having substantially the same size as a desired. thickness of the liquid crystal layer are formed on at least one of a pair of the substrates. Due to such a constitution, a liquid crystal display device which can make the so-called cell gap and the luminance in the display screen uniform can be obtained.
The liquid crystal display device comprises a liquid crystal panel which includes a pair of substrates, with at least one of them being transparent, at least two kinds of color filters of different colors for color display and a black matrix interposed between the respective color filters, which are formed on one of a pair of the substrates, a group of electrodes, which includes signal wirings and common wirings formed on the other of a pair of the transparent substrates, a layer made of a liquid crystal composition disposed between a pair of the substrates and having a desired dielectric anisotropy, an orientation control layer for causing the molecular arrangement of the layer of this liquid crystal composition to be arranged in a given direction, polarizing plates which are laminated while having their respective polarizing axes disposed perpendicular to a pair of the gubstrates, and drive means which applies drive voltages to the group of electrodes, wherein the liquid crystal display device includes an electrode arrangement structure where a group of the electrodes apply voltages mainly in parallel with an interface between the orientation control layer and the layer of liquid crystal composition, and columnar spacers containing particles having substantially the same size as a desired thickness of the liquid crystal layer are provided on at least one of the pair of substrates, and the dielectric characteristics and the conductivity characteristics of the columnar spacers containing the above-mentioned particles are set to be higher than those of the liquid crystal composition, and the columnar spacers are formed on a portion between the signal wiring and the common wiring disposed at a position hidden by the black matrix.
Due to such a constitution, the mechanical strength of, the columnar spacer is enhanced and the disturbance of the electric field due to the provision of the columnar spacers can be prevented and, hence, a transverse electric field liquid crystal display device which exhibits the high contrast and the high luminance and is free from crosstalk can be obtained. Accordingly, a transverse electric field liquid crystal display device which prevents a lowering of the aperture rate, exhibits a high contrast and a high luminance with a black matrix of relatively low optical density, and is free from crosstalk can be obtained.
The particles contained in the columnar spacers are made of conductive beads. Since the particles contained in the columnar spacers are made of conductive material, the electric characteristics of the columnar spacers can be easily changed by arbitrarily setting the resistance value and the dielectric constant of the particles, as well as the material of the columnar spacers, and, hence, spacers having a smaller resistance and a higher dielectric constant than the liquid crystal can be formed so as to concentrate the noise electric field to the spacers, thus reducing the influence of the domain.
The columnar spacers are made of the same material as a protection film formed on an upper layer of the color filter formed on one substrate. A protective film which is made of organic material containing conductive spacers is formed on the other substrate. Due to such a constitution, the columnar spacers and the protective film can be simultaneously formed and, hence, the spacers can be manufactured easily.